The present invention relates to a magnetic disk apparatus, and in particular, to a magnetoresistive head for use in reproducing magnetically recorded information.
With the introduction of a more compact and high-density magnetic disk apparatus, a magnetoresistive head (MR head) which is capable of producing a high reproducing output voltage independent of the relative speed between the disk and the head has been put to actual use. An MR head presently in use on a magnetic disk apparatus utilizes an anisotropic magnetoresistive effect in which the electric resistance of the head changes depending on the relative angle between the direction of magnetization in a magnetic film and the direction of the signal detection current flow. Efforts to enhance its performance are being made through improvements in head structure and use of thin film materials. When a high aereal recording density as high as several Gb/in.sup.2 is required, any MR head which utilizes the anisotropic magnetoresistive effect will not be able to achieve enough sensitivity; therefore, development of a new head utilizing a giant magnetoresistive effect is under way in which the electrical resistance of the head changes in response to the relative angle between respective directions of magnetization in two magnetic thin films which are laminated with a non-magnetic conductive thin film interposed therebetween. In any of the MR heads mentioned above, changes of electrical resistance take place due to rotation of magnetization in the magnetoresistance film; therefore, in order to obtain a noise-free reproducing waveform, movement of domain walls must be suppressed as much as possible.
As a means for suppressing Barkhausen noise due to movement of the domain walls, a laminated structure is disclosed in U.S. Pat. No. 5,005,096 in which a hard magnetic thin film is laminated on a magnetoresistance film via a non-magnetic thin film, and other structures are disclosed in U.S. Pat. Nos. 5,018,037 and 5,079,035 in which hard magnetic thin films are disposed in abutting relationship with the magneto-resistance film on both sides thereof.
For any hard magnetic thin film to be used in a magnetoresistive head, two fundamental magnetic properties are required in order to prevent Barkhausen noise. One requirement is that the head must have a large coercive force. Namely, since the MR head has applied thereto a signal field from a recorded medium and also is subjected to a recording field, in order to ensure that a stable reproducing characteristic will be maintained even when such external magnetic fields are applied, a coercive force of a sufficient magnitude is required so that a longitudinal bias magnetic field impressed from the hard magnetic thin film to the magnetoresistance film will not change easily. The other requirement is that an in-plane component of magnetization should be large enough, that is, the squareness of a hysteresis loop along an intraplane direction should be large. Since it is this component among the magnetization components of the hard magnetic thin film that plays a major role to act effectively as the longitudinal bias field, it is necessary for this in-plane component to be substantially large, as well as for the squareness of its hysteresis loop to be substantially large, so that the longitudinal bias field will remain invariant even if external magnetic fields are applied.
FIG. 15 is a schematic diagram indicative of the structure of a prior art MR head disclosed in U.S. Pat. No. 5,005,096. This prior art MR head is directed to suppressing Barkhausen noise in magnetoresistive film 15 by impressing thereon a magnetic field produced by hard magnetic thin film 26, which is formed on non-magnetic underlayer 251 made of Cr or the like. Although it is possible to obtain a hard magnetic thin film 26 which has a large coercive force and a large squareness through the provision of non-magnetic underlayer 251, since a portion of the magnetic field derived from hard magnetic thin film 26 is caused to recirculate through an MR element, including soft magnetic thin film 13, spacer film 14 and magnetoresistive film 15, the directions of magnetization in the magnetoresistive film 15 become opposite between the sensing region and both sides thereof, as indicated in FIG. 15. Therefore, the state of magnetization in the magnetoresistive film 15 becomes very unstable, with the result that it becomes difficult to suppress Barkhausen noise.
FIG. 16 depicts the structure of an MR element disclosed in U.S. Pat. Nos. 5,018,037 and 5,079,035, in which, on both sides of the MR element, a hard magnetic thin film is formed in order to eliminate a region having a magnetization component whose direction is reversed within the magnetoresistive film, and thereby ensure that the magnetic field generated from the hard magnetic thin film is caused to act only in a single direction. This structure of laminated films, including soft magnetic thin film 13, spacer film 14 and magnetoresistive film 15 (hereinafter referred to as soft magnetic thin film/spacer film/magnetoresistive film) is formed by the steps of etching other regions except for the sensing region, forming hard magnetic thin films 26 on the both sides of the sensing region, forming electrodes on the hard magnetic thin films 26.
Then, it is necessary to etch the sides of the lamination of the soft magnetic thin film/spacer film/magnetoresistive film to form a taper so as to obtain a magnetic coupling and electrical contact between the soft magnetic thin film/spacer film/magnetoresistive film and the hard magnetic thin film 26 as well as the electrode, whereby, a portion of hard magnetic thin film 26 is formed on the taper of the soft magnetic thin film/spacer film/magnetoresistive film. However, there is a problem associated with this prior art technique in that, since the soft magnetic thin film 13 or magnetoresistive film 15 normally has a crystal structure of a face-centered cubic lattice, a portion of the hard magnetic thin film which is formed on such a crystal structure tends to have its property deteriorate greatly, in particular, its coercive force, compared to that in other portions thereof.
Further, there is another problem associated with the prior art which generally uses a Co--Cr--Pt hard magnetic thin film or Co--Cr hard magnetic thin film as the hard magnetic thin film 26 in that it is difficult to obtain a sufficiently large in-plane component of magnetization or squareness on other areas excepting areas of the soft magnetic thin film 13 or magnetoresistive film 15. Growth of thin films generally has a tendency that the most dense crystal plane becomes parallel to the film surface, whereby, in the case of the hard magnetic thin film of the prior art, the &lt;001&gt; plane is likely to be oriented parallel to the film surface. On the other hand, since the direction of easy magnetization is in the direction of &lt;001&gt;, magnetization tends to be directed perpendicular to the film plane, which causes the in-plane component that generates the longitudinal bias field most effectively to decrease.
These problems associated with the prior art can be solved by providing an appropriate underlayer and by forming a hard magnetic thin film thereon. According to current studies on magnetic recording medium, the provision of non-magnetic underlayers made of Cr or the like is known to be effective. However, the non-magnetic underlayer provided under the hard magnetic thin film for use in the MR head will interrupt a mutual magnetic coupling between hard magnetic thin film 26 and soft magnetic thin film 13 as well as magnetoresistive film 15, whereby, a desired effect to stabilize magnetization in both side regions of the soft magnetic thin film 13 and magnetoresistive film 15 cannot be attained. Thereby, magnetization in these ferromagnetic thin films become unstable, causing a Barkhausen noise and a fluctuation in reproducing characteristics to occur readily.
While the problems associated with the prior art MR head using the anisotropic magnetoresistive effect have been described hereinabove, it should be understood that the same problems will take place with an MR head which uses a macro magnetoresistive effect, since its MR element is composed of a ferromagnetic thin film having a crystal structure of a face-centered cubic lattice.